Apparatus for dielectric heating



y 1969 w. A. CUMMING 3,457,385

APPARATUS FOR DIELECTRIC HEATING Filed July 7, 1966 2 Sheets-Sheet 1July 22, 1969* w. A. CUMMING 3,457,385

APPARATUS FOR DIELECTRIC HEATING Filed July v, 1966 2 Sheets-Sheet 2United States Patent US. Cl. 219--10.61 Claims ABSTRACT OF THEDISCLOSURE Microwave energy in the form of a single travelling wave ispropagated along a substantially electrically continuous waveguide, themicrowave being of an operating mode that yields a concentration oflines of electric force in a certain area of the cross-section of thewaveguide, through which area an elongated workpiece is fed along itsown longitudinal axis. For web-shaped workpieces, a waveguide ofrectangular cross-section is preferable, in which a microwave of the TEmode is excited, having a concentration of lines of electric force inthe longitudinal centre plane of the waveguide, whereas for workpiecesin the form of filaments a waveguide of circular cross-section is chosenin which a wave of the TM mode yields a concentration of lines ofelectric force along the central longitudinal axis of the waveguide.

This invention relates to improvements in apparatus for dielectricheating, that is to say the heating of materials by microwave energy. Inone particular application, the invention is concerned with the dryingof elongated articles by heating the water entrained therein by theapplication of microwave energy. Such articles may typically be webs ofmaterial, paper for example, or filaments such as are produced in thetextile industries. For convenience, the article that is to be subjectedto drying, or heating for any other purpose (including any entrainedsubstances, such as water) will be referred to below as the workpiece.

More specifically, the invention is concerned with the use of awaveguide for the application of mircowave energy to a workpiece.

It is fundamental to the art of dielectric drying that the basicmaterial of the workpiece should have a low loss tangent to themicrowave energy. On the other hand the frequency of the mircowaveenergy will be so chosen that water entrained in the material will havea high loss tangent at that frequency. The effect of these circumstancesis to cause relatively high absorption of the microwave energy by thewater and relatively low absorption of the mircowave energy by thematerial itself. This is the ideal way to evaporate water from aworkpiece, the heating energy being used at high efiiciency, while therisk of overheating the material itself is minimised.

-It is, however, important, if this theoretical efficiency is to berealised in practice, that the energy transfer be as complete aspossible. It is in respect of this aspect of the method that the variousprior art proposals for dielectric drying and heating generally have notbeen entirely satisfactory, and it is the principle object of thepresent invention to provide improved methods and apparatus forenhancing the energy transfer between microwave energy and workpiece.

Another object of the present invention is to achieve improvements inthe uniformity of heating of an elongated workpiece. Methods have beenproposed in the past for passing Wet webs of material throughwaveguides, while subjecting the material to microwave energy, butdifficulties have arisen in achieving a uniform drying effecttransversely of the web. This problem is especially acute when it isdesired to achieve a carefully controlled degree of partial drying. Aswill appear more fully from the description below, the method adopted inthe present invention is especially conducive to the achievement of auniform distribution of heating effect across the web, as well as alongthe web.

While the principal utilization envisaged for the present invention isthe drying of elongated webs of material, such as paper, or elongatedfilaments, such as are used in the textile industry, the invention isalso applicable to other applications where it is desired to heatelongated workpieces, and particularly elongated workpieces in which itis desired to heat one substance selectively, provided that theworkpiece is of a shape suitable for feeding along a waveguide. Forexample, the method can be used for curing adhesives, or for the settingplastics, always provided that the substance to be heated has asutiiciently high loss tangent at the frequency used to ensure that itabsorbs an appreciable amount of energy. The method can also be used fordrying pulpwoods.

Various manners in which the invention may be carried into practice areillustrated in the accompaying drawings. It is to be understood that thespecific forms of the invention illustrated in the drawings are providedby way of example only and not by way of limitation of the invention,the broad scope of which is defined in the appended claims.

In the drawings:

FIGURE 1 shows a perspective view of a waveguide illustrating a firstmethod of drying a Web of material;

FIGURE 2 shows a cut-away side view of a fragment of a modification ofFIGURE 1;

FIGURE 3 is a cut-away side view of a further alternative to FIGURE 1;

FIGURE 4 is a perspective cut-away view of yet another form ofwaveguide;

FIGURE 5 is a cut-away side view of the waveguide of FIGURE 4;

FIGURE 6 is an end view of a rectangular waveguide illustrating thelines of electric force in one mode of operation;

FIGURE 7 is a partly broken away view of structure illustrating a mannerof feeding microwave energy to a rectangular waveguide;

FIGURE 8 is a perspective view of a further form of the invention; thisform comprising a circular waveguide adapted for drying a filament-a1material;

FIGURE 9 is a diagrammatic sectional view of the circular waveguide ofFIGURE 8 illustrating the lines of electric force in one mode ofoperation thereof; and

FIGURE 10 is an end view of the waveguide of FIG- URES 8 and 9 taken onthe line X-X of FIGURE 9 and illustrating this view of the same lines ofelectric force.

FIGURE 1 shows a workpiece in the form of a web 12 of material, forexample paper, that can be fed between one roll 10 and a second roll 11by suitable driving means (not shown). The web 12 may be made to travelin either direction, the choice of the feed direction depending onvarious factors that will be more fully explained below. The web 12travels along the centre line of a rectangular waveguide that consistsof a main portion 13 and a pair of perpendicular side arms 14 from whichmicrowave energy is fed into the main waveguide portion 13.

FIGURE 2 shows a basic-ally similar construction in which the web 12again travels along a main waveguide portion 13. However, in this casethe arms 14a for feeding the mircowave energy into the main waveguide 13are inclined thereto, rather than being perpendicular to the plane ofthe web 12.

FIGURE 3 illustrates another variant of this construction in which theWeb 12 extends out of the waveguide 13 through a slot 15. Thisarrangement enables the microwave energy to be fed directly into the endof the waveguide 13 at its mouth 16.

Essential novel characteristics of the present invention, which will beseen to be embodied in all three forms thereof so far described, residein the facts that the waveguide 13 is substantially electricallycontinuous (for uniform propagation of microwave energy therealong) andthat the direction of propagation of the microwave energy along thewaveguide 13 is along the longitudinal axis of the web 12. In otherwords, regardless of the direction of movement of the web 12, which canbe either with or against the energy flow, the plane in which the weblies extends along the waveguide in the direction of energy propagation.This feature, which is in contrast to many prior art proposals in whicha Web is caused to travel across a waveguide transversely of thedirection of energy flow, is important in the achievement of uniformityin the heating action.

It is also essential to the invention that the waveguide is suitablyterminated, e.g. by means of conventional absorbing wedges 18. Thisavoids reflection of the microwave energy and the setting up of standingwaves. In other words the energy is propagated in a single travellingwave, in contrast to the double travelling wave that is the equivalentof a standing wave.

The preferred mode of energization of the waveguide 13 is the transverseelectric mode known as the TE mode (employing the usual United Statesnomenclature). FIGURE 6 shows a cross-section of the waveguide 13 withthe lines of electric force represented by arrows 17. The lines ofmagnetic force have not been illustrated, since they are of no interestto the present invention. The heating energy transfer is effected by theelectric field. The lines of electric force 17 shown in FIGURE 6 arethose of a rectangular waveguide stimulated in the TE mode which willnormally be the dominant mode. It will be noted that the lines 17 areconcentrated along the centre plane of the waveguide, that is to say,the plane extending across the greater dimensions of the waveguideequidistant from its upper and lower plates, i.e. centrally of theshorter dimension. For this reason, the plane of the web 12 is arrangedto lie generally along this centre plane of the waveguide, in order toobtain the result that a maximum number of lines of electric force liewithin the web 12. In reality the presence of the web 12 has the effectof further concentrating the lines of force, so that with the web 12present the concentration of lines of force in the centre plane is fargreater than can be illustrated in FIGURE 6.

While operation of the waveguide 13 in the TE mode is much preferred, itis nevertheless still within the scope of the invention to propagate themicrowave energy in a single travelling wave in one of the othertransverse electric modes, for example the TB or TE modes, or in acombination of the various transverse electric modes. For example, theTE mode could be used with advantage in a case where the apparatus wasused to dry one or more relatively narrow Webs, each defining a planeparallel to the shorter cross-sectional dimension of the waveguide,because in the TE mode the lines of electric force extend across theshorter dimension of the waveguide. In a square waveguide the TE and TEmodes are thus identical. Since one of the prime objects of the presentinvention is to achieve a high coupling between the microwave energy andthe workpiece, by causing a concentration of lines of electric force toextend along the workpiece itself (in contrast to cutting through itvertically or at an inclination), the mode or modes of propagationadopted will be chosen basically with a view to achieving thisobjective, having regard to the geometry of the waveguide and theworkpiece and the location and orientation of the latter in the former.

The preferred range of frequencies for the microwave energy is from 20cm. to 1 cm. Microwave energy in the region of 1 cm. is the mosteffective for drying puiposes,

because the peak in the loss curve for water occurs around 1 cm. On theother hand, energy sources are more readily available and are lessexpensive at lower frequencies, which considerations will usuallyencourage the use of the longer wavelengths. Another factor to beconsidered is that waveguides are larger for propagation of the longerwavelengths in the dominant mode. Thus, since the size of the waveguidewill often in practice be determined by the size of the web material,this factor may dictate the wavelength to be preferred, bearing in mindthat it will normally be convenient to operate under conditions in whichonly the dominant mode can propagate.

FIGURES 4 and 5 illustrate an alternative system in which the microwaveenergy is fed step-by-step into a main waveguide 20 along which the web12 passes in a manner similar to that of the Waveguide 13. The power isfed into a superposed waveguide 21 at its mouth 22 and is then leaked tothe operating waveguide 20 through slots 23. This arrangement achieves amore even lengthwise distribution of energy to the operating waveguide20 than in the previously described embodiments, and is especiallyuseful in the processing of webs having a very high attenuation of theenergy, for example a comparatively thick web. Such a thick web might,for instance, comprise one or more lengths of leather to be dried.

The maximum power absorption by the water in the material of the webwill tend to be in the power input region of the waveguide. For thisreason it will normally be preferred to arrange for the Web 12 to travelin the same direction as the energy. In this way a wet web entering thewaveguide adjacent the power input will be subjected to the major amountof drying energy. On the other hand, the already partly dried web at thefar end of the Waveguide will receive less energy, which is inaccordance with its requirement. Seldom in a drying operation is itdesired completely to dry the article, because this would tend to giverise to brittleness in the final product. The object is usually toprovide a controlled amount of drying, and this result can be achievedby suitable regulation of the input power in relation to the moisturecontent of the web and its rate of travel along the waveguide. Suchcontrol is usually more readily achieved when the direction of webtravel coincides with the direction of energy propagation.

In order to stimulate the waveguide 13 uniformly across its width, oneof the various known input devices may be employed. For example, aparabolic input (not shown) may be used. Alternatively, there may beused the known system illustrated in FIGURE 7 in which a probe 25 feedsmicrowave energy into an input waveguide 26 from which it is injectedinto the end of the main waveguide 13 uniformly across the width thereofthrough oppositely inclined slots 27. Similar injection means can beused to feed energy into the waveguides 14 and 14a. Wherever not shown,the waveguides may include conventional attenuating means, such asabsorbing wedges, located at their ends remote from the power inputs, inorder to absorb residual microwave energy.

FIGURES 8 to 10 show an application of the invention to the dielectricdrying of a filamental material 30 by means of microwave energy in acircular waveguide 31. Again, it is an essential feature of the presentinvention that the filament 30 extends along the waveguide in the samedirection as that of energy propagation. However, in the case of afilamental workpiece and a circular waveguide, it is preferred tooperate the waveguide in the transverse magnetic code known as the TMmode, in which the electric lines of force 32 are as shown in FIGURES 9and 10. In this case it is the portions of the lines of force 32 thatextend along the central axis of the waveguide, rather than those thatextend across the waveguide, that are exploited to obtain a highconcentration of heating effect (see the concentration of lines of forcedemonstrated at 33). The filament 30 is accordingly fed between suitablefeeding means (not shown), as far as practicable along this centralaxis. As in the previous examples, the natural concentration of lines ofelectric force along the centerline will be substantially increased bythe presence of the filament 30 itself. The direction of travel of thefilament 30 is optional, as before, but travel in the same direction asthe energy propagation is usually preferred, for the reasons alreadyexplained.

The TM mode, which is the preferred mode of propagation, is not thedominant mode in the waveguide 31. The dominant mode will be the TEmode. However, excitation in the TM mode with avoidance of excitation inthe TE mode can be achieved by the use of a pair of diagonally locatedinput probes 34, as shown in FIG- URES 8 and 9.

As will be apparent, a filamental workpiece is best handled in awaveguide that has so-called orthogonal symmetry, that is, a waveguideof a shape that can be turned through 90 about its longitudinal axis andyet remain basically the same in appearance, e.g. a circle, or a square,in contrast to a rectangular waveguide, for example.

Indeed the use of energy propagating in a transverse magnetic mode (notnecessarily mode TM and having a component of the electric field alongthe axis of the waveguide may be adopted for waveguide shapes other thanthose having orthogonal symmetry, e.g. rectangular waveguides. Such anarrangement might well have application to the heating of a flattenedfilament, or a relatively narrower but thick web, although the use ofthis arrangement is not restricted to any particular workpiece shape.

I claim:

1. Apparatus for subjecting a workpiece in the form of an elongated webto dielectric heating, comprising (a) a substantially electricallycontinuous waveguide of rectangular cross-section,

(b) means for moving said web along its own longitudinal plane along aselected plane in the waveguide, said selected plane extending along thewaveguide and across the greater cross-sectional dimension thereof at alocation substantially central of the shorter crosssectional dimensionthereof,

(c) and means for propagating microwave energy in either direction alongsaid waveguide principally in the TE mode to yield a concentration oflines of electric force in said selected plane occupied by the workpieceat a wavelength at which at least a part of said workpiece will absorbsaid energy to generate heat.

2. Apparatus according to claim 1 wherein said propagating meanscomprises means for injecting said energy into an end of the waveguidesubstantially uniformly thereacross.

3. Apparatus according to claim 2, wherein said propagating meanscomprises means for injecting said energy into said end of the waveguidesimultaneously from both sides of said workpiece.

4. Apparatus according to claim 2, wherein said means for moving theworkpiece along the waveguide includes means for introducing saidworkpiece through a side wall of the waveguide, and said propagatingmeans comprises means for injecting said energy directly endwise intosaid end of the waveguide.

5. Apparatus according to claim 1, wherein said propagating meanscomprises (h) a further similar rectangular waveguide superposed on themain waveguide along which the workpiece moves,

(i) means for injecting said energy into an end of said furtherWaveguide in said TE mode,

(j) and openings communicating between said waveguides, said openingsbeing arrayed across the width and along the length of said waveguideswhereby to transmit energy into said main waveguide gradually along thelength thereof and substantially uniformly across the width thereof.

References Cited UNITED STATES PATENTS 2,640,142 5/1953 Kinn 219-1061 X3,263,052 7/1966 Jeppson et al 2l910.55

3,307,010 2/1967 Piischner 219-10.61 X

3,321,314 5/1967 Jeppson 219-1055 X FOREIGN PATENTS 1,452,124 8/ 1966France.

1,471,131 1/1967 France.

OTHER REFERENCES Radar Electronics Fundamentals, Bureau of Ships, NavyDept. June 1944, pp. 364 to 368.

JOSEPH V. TRUHE, Primary Examiner L. H. BENDER, Assistant Examiner US.Cl. X.R. 219-1055

